Pool subscription limit

The value of using a subscription limit is to manage the maximum amount of over-provisioning that is acceptable for a pool. By managing the pool subscription limit, you can control the potential demand for storing data that might exceed the pool capacity.

olh-note.gif If the pool created by pool-VOLs assigned from accelerated compression-enabled parity groups is used, the pool subscription limit is defined with respect to the pool capacity not reserved for writing. In this case, the free area of the pool must be monitored even if the subscription limit is defined to 100%. If you don't want to monitor the free area of the pool, specify the subscription limit conforming to the following formula:

Pool capacity / Pool capacity reserved for data-writing x Subscription limit = 100%

For example, if 100 TB of the pool capacity and 80 TB of the pool capacity reserved for data-writing both exist, specify 80% as the subscription limit.

The subscription limit is the ratio (%) of the total DP-VOL capacity that has been configured to the total capacity of the pool. When the subscription limit is set, you cannot configure another DP-VOL if the new DP-VOL capacity would cause the subscription limit to be exceeded.

The subscription limit includes pages required to store user data and control information. The total capacity of DP-VOLs that are created from the pool is smaller than the subscription limit capacity. The formula used to calculate the required pages for one DP-VOL includes the control information. To determine the total pages required in a pool, multiply the number of calculated pages by the number of DP-VOLs. The value enclosed in ceil( ) must be rounded up to the nearest whole number.

Number of pages for the DP-VOL including the control information = 
ceil((One DP-VOL capacity (MB) + ceil(One DP-VOL capacity (MB) /
3,145,548 (MB)) * 4 (Pages) * 42 (MB)) / 42 (MB))

For example, if the pool capacity is 100 GB and the subscription limit is 150%, you can configure 150 GB of capacity to the DP-VOLs related to the pool.

The following figure depicts setting the subscription limit of pool capacity.

Monitoring total DP-VOL subscription for a pool

You can configure the Subscription Limit of total DP-VOL capacity to pool capacity. This prevents a new DP-VOL capacity that exceeds the configured subscription limit from being allocated and is associated with the pool. If you specify more than 100% as the subscription limit or the subscription limit is not set, monitor the free pool capacity because it is possible that writes to the DP-VOLs can exceed pool capacity. For details about the Subscription Limit, see Create Pools window.

The used value displayed on the cell for Current in the Subscription (%) is truncated after the decimal point of the calculated value. Therefore, the actual percentage of DP-VOL assigned to the pool can be larger than the value displayed on the window. If you create a new DP-VOL of the same size as the existing DP-VOL, the larger capacity size displayed on the Current cell is necessary.

For example, if 3 GB V-VOL is related to an 11.89 GB pool, the capacity (%) is calculated as follows:

(( ceil(3,072 (MB) / 3,145,548 (MB)) * 4 (Pages) * 42 (MB)) +
3,072 (MB)) / 12,175.36 (MB)) * 100 = 26.61....(%)

In this case, 26 (%) is displayed on the cell for Current in the Subscription (%). If you create a new V-VOL of the same size as the existing V-VOL, 27 (%) or more remaining capacity is necessary.